Combustion synthesis was used to prepare cobalt catalysts supported on alumina and silica-doped alumina for Fischer-Tropsch synthesis. Compared to catalysts made by conventional impregnation, the combustion-synthesized catalysts had smaller cobalt crystallite sizes of 17-18 nm, higher cobalt reduction levels over 92%, and higher metal dispersion of 16.1%. In Fischer-Tropsch testing, the combustion-synthesized catalysts produced 40-70% more C6+ hydrocarbons and formed longer hydrocarbon chain waxes compared to middle distillates, indicating higher chain growth probability. Silica-doped alumina as a support gave 12-13% higher C6+ yields than al

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Investigations into enhanced wax production
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DOI: 10.1016/j.enconman.2016.02.075
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2. Investigations into enhanced wax production with combustion
synthesized Fischer–Tropsch catalysts
Snehesh Shivananda Ail, S. Dasappa ⇑
Center for Sustainable Technologies, Indian Institute of Science, Bangalore 560012, India
a r t i c l e i n f o
Article history:
Received 25 November 2015
Accepted 26 February 2016
Keywords:
Fischer–Tropsch
Higher hydrocarbons
Liquid fuel
Combustion synthesis catalysts
Silica doped alumina
Metal support interaction
a b s t r a c t
Combustion synthesized (CS) cobalt catalysts deposited over two supports, alumina and silica doped alu-
mina (SDA), were characterized and tested for its Fischer–Tropsch (FT) activity. The properties of CS cat-
alysts were compared to catalysts synthesized by conventional impregnation method (IWI). The CS
catalysts resulted in 40–70% increase in the yield of C6+ hydrocarbons compared to IWI catalysts. The
FT activity for CS catalysts showed formation of long chain hydrocarbon waxes (C24+) compared to the
formation of middle distillates (C10–C20) for IWI synthesized catalysts, indicating higher hydrocarbon
chain growth probability for CS catalysts. This is ascribed to the smaller crystallite sizes, increased degree
of cobalt reduction and consequentially, a higher number of active metal sites exposed over the catalyst
surface. Additionally, 12–13% increase in the overall C6+ hydrocarbon yield is realized for SDA-CS cata-
lysts, compared to Al2O3-CS catalysts. The improved performance of CS-SDA catalysts is attributed to
48% increase in cobalt dispersion compared to Al2O3 supported CS catalysts, which is again caused by
the decrease in the cobalt-support interaction for SDA supports. The metal support interactions were ana-
lyzed using XPS and H2 TPR–TPD experiments. Combustion method produced catalysts with smaller crys-
tallite size (17–18 nm), higher degree of reduction ($92%) and higher metal dispersion (16.1%) compared
to the IWI method. Despite its enhanced properties, the CS catalysts require prominently higher reduc-
tion temperatures ($1100–1200 K). The hydrocarbon product analysis for Al2O3 supported catalyst
showed higher paraffin wax concentrations compared to SDA supported catalysts, due to the lower sur-
face basicity of Al2O3. This work reveals the impact of the CS catalysts and the nature of support on FT
activity and hydrocarbon product spectrum.
Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction
Escalating energy demands due to depleting petroleum sources
has renewed interests in Fischer–Tropsch (FT) process. FT synthesis
reaction is at the core of gas to liquid (GTL) process and can be used
to convert natural gas, coal or biomass to liquid transportation fuel
[1]. Recently, biomass to liquid (BTL) process has attracted much
attention for producing environment friendly, carbon neutral die-
sel from biomass [2]. In this process, biomass is subjected to gasi-
fication to generate syngas (mixture of CO and H2), followed by
conversion to higher hydrocarbons via the FT process [3,4]. The
hydrocarbon products are cracked to produce high quality diesel
that are characterized by high cetane number and are generally
devoid of sulfur, nitrogen or aromatic compounds [5]. High quality
diesel is obtained in large quantities (nearly 80 wt.% of the total
products) using cobalt catalysts in the low temperature FT (LTFT)
reaction. The LTFT reactors, operating in the temperature range
of 463–513 K, predominantly yield waxes as a major products.
The commercially operating FT reactors maximize the production
of diesel by mild hydrocracking of waxes. The major advantage
of this particular process is that the wax can be hydrocracked com-
pletely to diesel, yielding nearly 80% diesel fractions. Moreover, the
diesel obtained by hydrocracking of waxes has higher degree of
branching that gives it an advantageous lower pour point and also
has nearly zero aromatic content, rendering it a high quality fuel
[6]. Leckel [7] described a process scheme that maximizes this high
quality diesel yield from an LTFT reactor, wherein nearly two-
thirds of the final diesel is obtained from the wax hydrocracker
while the remaining one-third is obtained from the fractionation
of the hydrotreated LTFT condensation fraction. Overall, the FT
products range from low molecular weight hydrocarbons to long
chain waxes. The products obtained are a direct function of
reaction conditions (temperature, pressure, space velocity) and
catalytic properties.
http://dx.doi.org/10.1016/j.enconman.2016.02.075
0196-8904/Ó 2016 Elsevier Ltd. All rights reserved.
⇑ Corresponding author. Tel.: +91 80 23600536; fax: +91 80 23601692.
E-mail addresses: snehesh@cgpl.iisc.ernet.in, snehesh.s@gmail.com (S.S. Ail),
dasappa@cgpl.iisc.ernet.in (S. Dasappa).
Energy Conversion and Management 116 (2016) 80–90
Contents lists available at ScienceDirect
Energy Conversion and Management
journal homepage: www.elsevier.com/locate/enconman

3. Catalyst synthesis procedure and choice of supports have a
broad effect on FT activity and selectivity. Several supports like
Al2O3, SiO2, TiO2, alumino-silicates and carbon nanofibers have
been investigated for its effect on FT activity [8–11]. Though the
fundamental operation of a support is just of physical nature, i.e.
to increase the amount of surface area available for reactants, very
often the dispersed metals show a strong affinity for the support,
eventually altering the catalytic activity [12]. FT active metals,
cobalt and iron, deposited over oxide supports show a distinct
metal-support interaction and depending on the nature of this
interaction FT activity is dynamically affected. Detailed work on
the influence of choice of support on FT reaction has been given
by Storsæter et al. [13]. The extent of metal reduction was shown
to be the least for Al2O3 supported catalysts, and highest for SiO2
supported catalysts, indicating a stronger metal support interac-
tion for Al2O3 compared to SiO2 supported catalysts. For cobalt–
alumina system, the formation of cobalt aluminate species over
Al2O3 supports further reduce the available active sites for FT syn-
thesis [13]. Apart from the formation of bulk aluminates over Al2O3
supports, the degree of reduction and hence the metal dispersion
reduces due to the formation of Co2+Co3þ
2ÀxAlxO4 mixed spinels,
additionally reducing the FT activity [14,15].
More recently, several researchers have investigated the prop-
erties of FT active metals supported over silica doped alumina
(SDA). Catalysts characterized by Jean-Marie et al. [16], showed
lower concentrations of cobalt aluminate compounds in SDA com-
pared to alumina supported catalysts. The use of SDA support with
composition of 5% SiO2 and 95% Al2O3, significantly enhanced the
cobalt reducibility, resulting in 18% increase in CO conversion
[16]. The work by Daniell et al. [17] reported formation of Lewis
and Bronsted acid sites by addition of SiO2 to c-Al2O3. Moreover,
the quantity and the strength of these acid sites were highest for
40% SiO2 weight fraction in the synthesized supports [17]. Conse-
quently, the amount of acid sites has a direct influence on the FT
product spectrum. Low acidity catalysts produce hydrocarbon
products comprising mostly straight chain alkenes and alkanes.
More acidic catalysts result in FT products comprising lighter and
more branched hydrocarbons [18].
The catalyst synthesis conditions display pronounced effect on
the FT activity, hydrocarbon selectivity and product distribution.
Catalyst synthesis involves introduction of metal precursor into
supports of high surface area and calcination of the deposited
metal salt into its oxide form. For the FT reaction, the metal oxide
is further reduced to its zero valent form by H2 reduction step.
Addition of metal salts into the catalyst carrier is achieved most
commonly by incipient wetness impregnation (IWI) method and
co-precipitation method. The combustion synthesis (CS) method
of catalyst preparation has been used for synthesis of large number
of metal oxides [19–29] and can be applied for synthesizing sup-
ported cobalt catalysts for FT reactions. The CS method, also
referred to as self-propagating high temperature synthesis, is an
efficient approach for developing metal oxides, wherein the heat
required for the decomposition of the metal precursors to its cor-
responding metal oxides is generated by the exothermic redox
reaction of a fuel and an oxidizer. The synthesis process includes
impregnation of a mixture of fuel (urea, citric acid, hydrazine,
etc.) and oxidizer (active-metal salts like cobalt nitrate) into the
support extrudes and ignition of this mixture by an external heat-
ing source. The combustion of this mixture generates a high tem-
perature reaction wave across the volume causing decomposition
of the mixture precursors and resulting in the formation of metal
oxides. The major limitation for CS process is the high heat release
and the consequential high temperature rise rates ($200–300 K/
min), resulting in the evolution of combustion products with
uncontrolled explosion, eventually powdering the catalysts and
in most cases resulting in the loss of active components. Such vig-
orous behavior of CS reaction is distinctly evident for metal loading
>8–10% [22]. The use of these powdered catalysts in a fixed bed
reactor demands either re-pelletizing or re-molding which is con-
sidered very disadvantageous [30].
Shi et al. [31] synthesized SiO2 supported Co catalysts by com-
bustion synthesis and reported a CO conversion of 46.5% compared
to 16.7% for IWI synthesized catalysts. The increased CO conversion
was primarily attributed to reduced crystallite size, increased
degree of reduction and higher metal dispersion for CS catalysts.
In another work, FT active SiO2 catalysts were synthesized by Shi
et al. [32] in a single step without the need for further reduction.
This was achieved by maintaining an equivalence ratio (the ratio
of the actual fuel-to-oxidizer ratio to the stoichiometric fuel-to-
oxidizer ratio) of 1.5 and a non-oxidizing environment during the
combustion step. Though a remarkable method of synthesizing
active catalyst, the authors in their work have not reported the cru-
cial information indicating the effect on hydrocarbon product
selectivity or product distribution. LeViness et al. [33], in their
work reported the use of silica cobalt catalysts synthesized by
the proprietary organic matrix combustion process, used in a
microchannel reactor. The authors reported an average CO conver-
sion of 70%, CH4 selectivity of 7%, and a C5+ selectivity of 88%. The
productive reaction performance was achieved by improved cata-
lyst properties and efficient reactor performance.
In the present study, the combustion synthesis of 20 wt.% cobalt
on alumina and, silica doped alumina support extrudes has been
explored. The synthesized catalysts are characterized to under-
stand the impact on metal support interaction as a consequence
of the synthesis mechanism and its influence on the FT activity
and product selectivity. Catalysts are characterized using DTA–
TGA, FTIR, TPR–TPD, XRD, and XPS. The investigations conducted
confirm the influence of support and the role combustion synthesis
method on the hydrocarbon chain length formed. Properties of CS
catalysts and its FT activity are compared with the catalysts syn-
thesized by IWI method.
2. Experimental procedure
2.1. Catalyst synthesis
The redox mixture comprised of cobalt nitrate hexahydrate (Co
(NO3)2Á6H2O, Merck emplura) and hexamethylenetetramine
(C6H12N4, Alfa Aesar) as metal and fuel precursors respectively.
The supports, c-alumina and silica doped alumina (40 wt.%
SiO2/60 wt.% Al2O3) comprised of 1 mm spheres with a surface area
of 158 m2
/g and 380 m2
/g respectively. The aqueous solution of
metal and fuel precursors was impregnated in the supports by stir-
ring in a rotary vacuum evaporator. Cobalt loading for CS catalysts
was attained in multiple steps. IWI catalysts were synthesized by
calcining the impregnated support in a furnace maintained at
823 K for a duration of 6 h.
The stoichiometry for the redox reaction was selected with the
equivalence ratio of one, to achieve maximum heat release. As
described by Eq. (1), the metal nitrate to fuel molar ratio is 3.86.
The impregnated catalyst spheres were calcined in a muffle fur-
nace, and the temperature increased to 673 K at 10 K/min. To mea-
sure the combustion reaction temperature, a known mass of the
redox-mixture impregnated support is placed in a beaker and
heated in a muffle furnace. An R-type (1 mm diameter) thermocou-
ple is inserted into this beaker which contains the impregnated
support. The reaction temperature is recorded using a P-Daq data
acquisition system. Further, the redox mixture is heated by
increasing the furnace temperature at the rate of 10 K/min using
a PID controller. Once the reaction gets initiated, a reaction wave
S.S. Ail, S. Dasappa / Energy Conversion and Management 116 (2016) 80–90 81

4. spans throughout the support volume forming cobalt oxide. The
theoretical estimation of the maximum temperature achieved dur-
ing the combustion reaction was estimated by the method
described in Turns [34].
27CoðNO3Þ2 Á6H2Oþ7C6H12N4 ! 9Co3O4 þ41N2 þ204H2Oþ42CO2
ð1Þ
2.2. Catalyst characterization
The FT-IR spectra of impregnated supports and calcined cata-
lysts are examined to investigate the nature of the redox complex
and to probe if any combustion residues are remaining on the sup-
port surface. The samples are scanned from 4000 cmÀ1
to 400 cmÀ1
with a resolution of 4 cmÀ1
using a Perkin Elmer 100 FT-IR spec-
trometer coupled with ATR accessory.
The combustion thermal events are characterized with Perkin
Elmer STA-6000 analyzer to obtain the real-time measurements
and analysis of sample weight change and heat flow. The sample
is heated at the rate of 5 K/min in static air starting from initial
temperature of 313 K to a maximum temperature until no weight
loss is observed (dm/dt = 0), indicating completion of the combus-
tion reaction.
X-ray diffraction spectra were recorded by a Rigaku Smartlab X-
ray spectrometer to identify to cobalt oxide phases in the synthe-
sized catalysts. The 2-h values were scanned at a rate of 0.2°/min
from 30° to 70° with Cu Ka radiation (k = 1.54 Å). Scherrer formula
was used to calculate the crystalline sizes of the synthesized cata-
lysts from the line broadening. Instrumental line broadening is
evaluated using the XRD spectra of Si-polycrystalline standard
and peak broadening due to strain is neglected. Effective peak
broadening is then incorporated into the Scherrer equation to
obtain the crystallite size.
Temperature programmed reduction and desorption (TPX,
X = reduction/desorption) studies are performed in a Micrometrics
Autochem 2920 TPX system. The analysis of obtained TPD profiles
gives the total number of active cobalt sites on the support surface,
and also provide the nature of metal support interaction. The
methods applied by Sewell et al. [35] for evaluation of degree of
reduction (DoR(%)) and metal dispersion (D(%)) have been used
as described in Eqs. (2) and (3).
DoRð%Þ ¼
NH2Àads à 58:93
mcat à L à SF
à 100 ð2Þ
Dð%Þ ¼
NH2Àdes à 2 à 58:93 à 104
L à mcat à DoR
à 100 ð3Þ
Here, NH2Àads is the H2 uptake from the TPR spectra, L is the per-
centage metal loading, SF is the Co0
reduction stoichiometric factor
(1.33), mcat is the mass of catalyst used for the TPR and TPD pro-
cess. Approximately 50 mg of sample is weighed and placed in a
U-shaped quartz cell. All TPX studies include conditioning of the
sample by heating in Ar at 473 K for two hours, to remove any
associated moisture traces. The quartz cell is cooled to 373 K and
the gas line switched to 10% (vol%) H2/Ar with a flow rate of
10 ml/min. The furnace temperature is increased to 1273 K at the
rate of 10 K/min. H2 consumption is measured using a TCD detec-
tor. The amount of chemisorbed H2 is measured by reducing
$30 mg of catalyst in pure H2 (purity >99.995%) for 16 h and then
cooled to 373 K. At this temperature, the gas flow is switched to Ar,
and the flow is maintained for one hour to remove physisorbed H2.
The catalyst cell is then ramped to 773 K at a rate of 20 K/min in Ar
flow to desorb the chemisorbed H2. The recorded TPD spectrum is
analyzed to quantify the chemisorbed H2. Total active sites and the
cobalt dispersion are calculated assuming dissociative H2
chemisorption with stoichiometry factor of 0.5.
NH3-TPD data were obtained to determine the surface acidity
and the total number of acid sites on the supports and synthesized
catalysts. Around 30 mg of sample is placed in the quartz sample
holder and saturated with NH3 at 373 K for two hours. The gas flow
is then switched to Ar with a flow rate of 50 ml/min at the same
temperature for an hour to remove physisorbed NH3. The sample
temperature was then increased to 700 K at 20 K/min. The amount
of desorbed NH3 is recorded to determine the number of acidic
sites.
XPS information is obtained on AXIS ULTRA system with Al Ka
radiation (hm = 1486.6 eV), operated at 15 kV, 10 mA and ambient
temperature with the vacuum of 10À8
Torr. The narrow region of
core level spectra of the calcined and reduced catalysts are
analyzed to obtain information on the cobalt oxidation state.
The XPS spectrum peak shift is corrected using the binding energy
(B.E.) of C-1s spectra observed at 284.8 eV. The raw XPS data are
deconvoluted using CasaXPS software to separate the contribu-
tions from various oxide phases.
2.3. Fischer–Tropsch activity studies
FT experiments are performed in a fixed bed reactor at 3 MPa
and 503 K. Experimental layout involved in this work is shown in
Fig. 1. The syngas feed consists of 70% H2 and 30% CO volumetric
gas composition. The inlet gas flow rate of is controlled by Brooks
mass flow controller and the pressure in the reactor is regulated
using TESCOM ER-5000 BPR. Catalyst bed temperature is measured
using an axially inserted resistance temperature detector (RTD)
and the reactor temperature is controlled using a proportional–inte
gral–derivative (PID) controller. The synthesized catalysts, prior to
FT reactions, are reduced in H2 (purity = 99.995%), at a temperature
of 1200 K, for a duration of 20 h. Six grams of reduced catalysts are
mixed with 25 g of bSiC, to maintain isothermal bed conditions,
and transferred carefully into the FT reactor. Gas composition
and exit gas flow rates are recorded for a duration of 85 h, after a
stabilization period of 24 h. Exit gas composition is measured using
an online Perkin Elmer Clarus 680 gas chromatograph with an
elite-Alumina column. The product transfer line connecting the exit
of the FT reactor to the condenser is maintained at 423 K to prevent
the condensation of any hydrocarbon products. The higher
hydrocarbons are separated in a gas–liquid separator consisting
of a cold trap maintained at 273 K and a hot trap maintained at
373 K. Liquid samples are analyzed, after every run in a Perkin
Elmer Clarus SQ8C-MS. Eqs. (4) and (5) evaluate the percentage
CO conversion (vCO(%)) and percentage hydrocarbon selectivity
(SCHÃ(%)). Here, mÃ
COÀin, mÃ
COÀout are the mass flow rates of CO enter-
ing and leaving the system and mÃ
CHÃ is the mass flow rate of the
hydrocarbon product specie.
vCOð%Þ ¼
mÃ
COÀin À mÃ
COÀout
mÃ
COÀin
à 100 ð4Þ
SCHÃð%Þ ¼
mÃ
CHÃ
mÃ
COÀin À mÃ
COÀout
à 100 ð5Þ
3. Results and discussions
This section details the catalyst characterization results,
indicating the redox combustion properties for the selected CS sto-
ichiometry, the reduction behavior of cobalt oxides, the cobalt
crystallite size and the nature of metal support interaction for
82 S.S. Ail, S. Dasappa / Energy Conversion and Management 116 (2016) 80–90

5. the synthesized catalysts. Further, the section describes the effect
of the developed catalysts on the Fischer–Tropsch reaction.
3.1. Catalyst characterization
3.1.1. Combustion thermodynamics
CS catalysts are synthesized with a reaction stoichiometry such
that the impregnated redox mixture react over the support surface
without any residual traces of carbonaceous products or unburnt
fuel and metal precursors, resulting only in the formation of cobalt
oxide. Hence, the fuel-to-oxidizer ratio should be such that, suffi-
cient energy is generated in a self-sustained state for the complete
decomposition of fuel and metal precursor into cobalt oxide and
gaseous products, viz., CO2, N2, and H2O. Maximum theoretical
temperature of 1457 K was obtained for the selected redox reac-
tion using the thermodynamic data listed in the Supplementary
Material link, identified as Table SM-A.
Fig. 2 shows the temperature–time profile for hexam-
ethylenetetramine (HMTA) and cobalt nitrate redox reaction with
an oxidizer to fuel molar ratio of 3.86. Two stages of combustion
are observed. The first stage, observed from the reaction initiation
at 421 K to 480 K with a slope of 15 K/min, is due to the
commencement of surface combustion reaction. The second stage,
as observed from 480 K to a maximum temperature of 571 K, is
accounted for the redox reaction of bulk NOÀ
3 and the fuel with
the temperature rise rate of 180–200 K/min.
CS reactions are marked by rapid heat release rates and large
rates of product formation, resulting in an uncontrolled combus-
tion process, especially for metal loading greater than 5% and even-
tually forming powdered catalysts. This factor has prevented
extensive use of CS for catalyst synthesis, especially for reactions
like Fischer–Tropsch, which require metal loading >15–20%
[36,37,11,38]. In the current study, catalysts with metal loading
of 20% have been achieved on spherical supports for direct use in
a fixed bed reactor.
The thermal events that occur in the course of redox reaction
were recorded using a DT–TGA analyzer. Fig. 3 shows the DT–
TGA profile for CS and IWI impregnated catalysts. The redox reac-
tions occur in the temperature range of 430–520 K, where a total
weight loss of 25% is observed. Region I for CS catalysts, as seen
from 430 K to 455 K, shows 4.5% sample weight loss. The exother-
mic reactions that occur in this region correspond to the surface
combustion of NOÀ
3 and HMTA. In region II, observed from 455 K
to 475 K, the sharp exothermic peak corresponds to the redox reac-
Fig. 1. Fischer–Tropsch activity test setup.
20 40 60
350
400
450
500
550
600
T-start = 421 K
Temperature(K)
Time (min)
T-max = 571 K
Fig. 2. Temperature time profile for HMTA-Co(NO3)2Á6H2O reaction.
0
-10
I
IIIIII
CS
50
60
70
80
90
400 450 500 550 600
0
-10
DTA
TGA
Temperature (K)
Heatflow,endodown(mW)
IWI
DTA
TGA
60
70
80
90
Weightloss(%)
Fig. 3. Simultaneous thermogravimetric and differential thermal analysis for CS
and IWI catalysts.
S.S. Ail, S. Dasappa / Energy Conversion and Management 116 (2016) 80–90 83

6. tion of bulk NOÀ
3 and HMTA. A maximum weight loss of 16% is
observed in this region. The weight loss observed in region III is
due to the redox reaction of residual fuel–oxidizer mixture, as
observed from 476 K to 521 K, accounting for 5% sample weight
loss. The thermal behavior observed for IWI catalysts shows a
broad weight loss region with an endothermic decomposition of
Co(NO3)2Á6H2O, resulting in the formation Co3O4. The weight loss
curve for CS catalysts shows clearly that the selected stoichiometry
produces enough heat for decomposition of cobalt nitrate resulting
in the formation of supported Co3O4.
Fourier transform infrared (FT-IR) spectroscopy of the fuel–ox-
idizer mixture and the burnt catalysts were examined to primarily
investigate the nature of the redox complex and to probe the com-
pletion of the combustion reaction. Fig. 4 shows the FT-IR spec-
trum of the impregnated redox mixture and the calcined
catalysts. The distinct peak observed at 1360 cm1
, which splits at
1313 cm1
and 1296 cm1
, corresponds to the symmetric stretching
of mono-dentate nitrate ion. The split band spectra indicate the
coordination of HMTA to the Co ion. HMTA, a tetradentate ligand,
complexes with metal ions and acts as a combustion precursor
for the complex metal ions [24]. The broad peak observed at
3352 cm1
indicates the presence of moisture in the sample and
the peak identified at 819 cm1
corresponds to the vibration spectra
of CH2 rocking. For calcined catalysts, the FT-IR spectra show no
existence of the redox precursors or the combustion residues, fur-
ther supporting complete combustion of the redox mixture.
3.1.2. Temperature programmed reduction and desorption
Fig. 5 shows the H2 reduction profile for the synthesized cata-
lysts. Primary observations, as seen from the plots, reveal that
the synthesis procedure and the nature of support used, have a
profound effect on the catalyst reduction temperatures as well as
the degree of reduction. For IWI synthesized SDA supported cata-
lysts, H2 reduction peaks are observed at 547 K, 611 K and 909 K.
The initial two peaks correspond to the twin-step reduction of
Co3O4 (Co3O4 À! CoO À! Co), whereas the broad peak observed
at the higher temperature are due to reduction of various phases
of cobalt oxides formed on the support surface. The reduction of
various phases of cobalt oxides supported on Al2O3 was investi-
gated by Arnoldy and Moulijn [39], where four stages of cobalt
oxide were reported, corresponding to the four different types of
cobalt phases on Al2O3. Phase-1 (T1 = 550–580 K) corresponds to
the twin step reduction of Co3O4, as described earlier. Phase-2, as
described by Arnoldy and Moulijn [39], represents Co3+
ions in Co3-
AlO6 crystallites, with reduction peak at 750 K. For CS catalysts,
minimal H2 uptake is observed, though some traces of H2 con-
sumption are observed for IWI catalysts. Phase-3 consists of the
reduction of surface Co2+
ions, as observed at 900 K. A strong peak
for Al2O3-CS is observed at 915 K (T3), clearly indicating the reduc-
tion of surface Co2+
ions. Phase-4 as expressed by Arnoldy and
Moulijn [39], at temperature $1200 K, includes the reduction of
surface or subsurface Co2+
ions in the Co2+
–Al3+
spinel structure.
From the TPR profile observed in Fig. 5, it is evident that the spinel
structures continue to get reduced at temperatures above 1000 K.
H2 consumption is not observed for IWI catalysts above 1000 K.
The irreducible cobalt oxide species that reduce at temperatures
above 1200 K have been reported by Schanke et al. [40], Jabłon´ ski
et al. [41], Arnoldy and Moulijn [39]. It is likely that these species
are present in IWI synthesized catalysts which occur in a stable
form at low temperatures [42].
The reduction temperatures of the synthesized catalysts, the
degree of reduction and the cobalt dispersion are tabulated in
Table 1. As seen from the table, the degree of reduction for IWI syn-
thesized, silica doped alumina catalysts is higher than Al2O3 sup-
ported catalysts by 27%. The higher degree of reduction for SDA
supported cobalt catalysts is due to the reduced formation of
cobalt aluminates. Alteration of the Al2O3 support surface by the
addition of SiO2 results in the formation of alumino-silicates that
impede the formation of barely reducible cobalt aluminates. It is
evident from Table 1 that the total fraction of active Co0
sites
increases to 12.1% for IWI-SDA supported catalysts from 8.1% for
IWI-Al2O3 supported catalysts, indicating a 41% increase in the
metal dispersion. In case of SDA supported catalysts, the dispersion
Fig. 4. FT-IR of supported redox precursors and calcined catalysts.
Fig. 5. Temperature programmed reduction of IWI and CS catalysts.
Table 1
TPR–TPD summary.
Catalyst Reduction temperatures DOR (%)a
D (%)b
T1 (K) T2 (K) T3 (K)
SDA-IWI 547 611 909 81 12.1
SDA-CS 557 – 986 92 16.1
Al2O3-IWI 581 801 – 63.85 8.6
Al2O3-CS 479 558 913; 1140 68.72 10.9
Co3O4 512 574 – – –
a
From TPR H2 uptake.
b
From H2 TPD of reduced catalysts.
84 S.S. Ail, S. Dasappa / Energy Conversion and Management 116 (2016) 80–90

7. increases from 12.1% for IWI catalysts to 16.1% for CS catalysts and,
for Al2O3 supported catalysts, the dispersion increases from 8.6%
for IWI catalysts to 10.9% for CS catalysts. These results evidence
higher fraction of active Co0
over combustion synthesized cata-
lysts. Combustion synthesized Co/SiO2 catalysts reported by Shi
et al. [32], showed increased reduction temperatures with the
degree of reduction of 89% and an improved metal dispersion of
16.6% compared to a degree of reduction of 82% and metal disper-
sion of 3.1% for IWI synthesized catalysts.
Table 2 gives the magnitude of acid sites expressed as mmol
NH3/gcat. As seen in the table, the SDA supported catalysts exhibit
higher surface acidity compared to the Al2O3 supported catalysts.
Daniell et al. [17], in their work, show the formation of highly
acidic Lewis and Bronsted acid sites for silica doped alumina sup-
ports, with properties similar to those found in zeolites. Increased
surface acidity is evident from higher NH3 desorption from the cat-
alyst surface.
3.1.3. X-ray diffraction
XRD pattern for the calcined catalysts showed distinct peaks
corresponding to Co3O4. Figs. 6 and 7 show the XRD spectra for
the calcined and reduced catalysts. The 2-h peaks observed at
31.2
, 36.94
, 44.86
, 55.82
, 59.24
and 65.16
are consistent with
the spectra of Co3O4. In case of Al2O3 supported catalysts, an addi-
tional peaks at 66.9
and a broad peak at 45.8
, that nearly merges
with the 44.86
peak, indicate the presence of c-alumina (ICSD-
40029; ICSD-95302 [43,44]). Al2O3 supported cobalt catalysts usu-
ally result in the formation of cobalt aluminates. However, due to
the close and overlapping signatures of CoAl2O4 and Co3O4, it is
extremely difficult to differentiate between the two structures
using XRD alone. While comparing with the Al2O3 supported cata-
lysts, minimum signatures of c-Al2O3 are still observed in the SDA
supported catalysts. Daniell et al. [17] in their work reported a sil-
ica encapsulation of Al2O3 for silica doping in the range of 30–60%.
From the XRD results of the SDA supported cobalt catalysts, it is
evident that minimum fractions of c-Al2O3 phase still remain
exposed on the catalyst surface. The XRD spectra of reduced cata-
lysts show sharp 2-h peaks at 44.2
and 51.5
, which correspond to
metallic Co(ICSD-53805 [45]). Furthermore, the Al2O3 supported
reduced cobalt catalysts shows a low intensity peak at 67.24
,
which corresponds to c-Al2O3. The absence of this peak in SDA sup-
ported catalysts, further evidences the SiO2 surface coverage.
Table 3 shows the Co3O4 and Co0
crystallite sizes for the synthe-
sized catalysts. The cobalt crystallite size decreases by a margin
of 30% for CS catalysts. The reduced crystallite size agrees with
the dispersion estimate obtained from H2 chemisorption experi-
ments. A possible interpretation of the reduced crystallite size of
CS catalysts can be obtained from the temperature–time experi-
ments of the CS catalyst. As observed in Fig. 2, the CS catalysts
are synthesized in a short span of 12–14 min. Contrarily, the IWI
catalysts require a calcination time of 3–4 h for the complete
decomposition of cobalt nitrate to cobalt oxide. The elongated cal-
cination time for IWI catalysts is a probable cause for the sintering
of cobalt crystallites resulting in catalysts with higher crystallite
size compared to CS catalysts. Strikingly, the reduced catalysts
were characterized by higher crystallite size, which is a likely out-
come of the increased reduction temperature, since the crystallite
size is expected to increase with temperature at constant pressure
[46,47]. The crystallite size may increase by diffusion of mass from
the outer layers of neighboring grains [46].
3.1.4. X-ray photoelectron spectroscopy
Figs. 8 and 9 show the deconvoluted spectra of the calcined cat-
alysts. Decomposition of the XPS peaks enables the identification
and quantification of the cobalt oxide phases present on the cata-
lyst surface. Spectral mapping of the high resolution elemental
spectra was obtained by Gaussian curve fitting routine, and the cor-
responding binding energy (B.E.) reference was obtained from the
Table 2
Surface acidity measured by NH3-TPD.
Sample Acid strength (mmol/gcat)
SDA 0.711
Al2O3 0.371
SDA-IWI 1.080
SDA-CS 1.230
Al2O3-IWI 0.576
Al2O3-CS 0.727
Fig. 6. XRD spectra of 20Co/SDA.
Fig. 7. XRD spectra of 20Co/Al2O3.
Table 3
Crystallite sizes of the SDA and Al2O3 supported catalysts.
Catalyst dCo3O4 (nm)a
dCo (nm)b
SDA-IWI 14.1 26
SDA-CS 10.7 18.2
Al2O3-IWI 15.0 22
Al2O3-CS 10.5 17.6
a
From XRD of calcined catalysts.
b
From XRD of reduced catalysts.
S.S. Ail, S. Dasappa / Energy Conversion and Management 116 (2016) 80–90 85

8. NIST database [48]. XPS spectra of Co3O4 standard was used to
identify the binding energy (B.E.) of Co3+
and Co2+
oxidation states.
The Co-2p3/2 and the spin orbital splitting B.E. are listed in Table 4.
The survey spectra of the Al2O3-IWI, Al2O3-CS, SDA-IWI and SDA-
CS are shown in Supplementary material link, identified as
Figure SM-I, SM-II, SM-III and SM-IV, respectively. The component
at 779.9 eV for the deconvoluted spectra is referenced to Co3O4,
which is the most dominant phase observed in the calcined
catalysts. XRD of the calcined catalysts too revealed Co3O4 as the
dominant phase. The second low intensity peak of the deconvo-
luted spectra corresponds to CoO. The satellite peaks observed at
783 eV, are indicative of CoO, and occur due to the charge transfer
band structure, assigned to the late 3d transition metal oxides [49].
The information on the nature of the metal support interaction can
be substantially obtained from the intensities of its satellite peaks.
A higher concentration of the cobalt aluminates is indicated by the
higher intensity of the shake-up satellites. This is because, CoAl2O4
has characteristics of strong shake-up satellites and has binding
energies 6–7 eV higher than the main peak. Ji et al. [50] and Zsol-
dos et al. [51] reported stronger satellite intensity of CoO and
CoAl2O4 specifying a stronger metal support interaction. The ratio
of the intensities of Co-2p3/2 peak at 779.9 eV to the intensity of
the shake-up satellite peak points out the relative formation of
CoAl2O4 in the synthesized catalysts. From Table 4, it is evident
that the silica doping in Al2O3 drastically reduces the cobalt alumi-
nate formation. Moreover, CS catalysts too revealed reduced alumi-
nate formation, implying a lower metal support interaction
compared to IWI catalysts.
The measure of the cobalt dispersion can be obtained from the
Al and Co peak intensity ratios [52,53]. Increasing intensity ratios
indicate higher cobalt dispersion, revealing lower metal
agglomeration. The percentage metal dispersion obtained from
H2 TPR–TPD experiments agree with the values obtained from
XPS spectrum analysis. The metal dispersion as observed from
the XPS spectral analysis shows that DÀAl2O3—IWI 6 DÀAl2O3—CS and
DÀSDA—IWI 6 DÀSDA—CS, further evidencing that the combustion
synthesized catalysts result in higher degree of reduction, higher
metal dispersion and consequently, larger number of active metal
sites for the FT reaction. Additionally, the SDA supports display
higher metal dispersion compared to alumina supported catalysts,
signifying reduced metal support interaction for SDA supported
catalyst.
3.2. FT activity and selectivity
The effect of the synthesized catalysts is shown in Fig. 10, where
the activity and selectivity measurements were averaged over a
duration of 85 h of operation. The synthesis procedure greatly
enhances the FT activity and selectivity, resulting in higher CO con-
version and C6+ selectivity for combustion synthesized catalysts,
compared to conventionally impregnated catalysts. The CO conver-
sion for SDA-CS catalysts increased to 42.9% from 33% as seen for
SDA-IWI catalysts. A similar trend is observed for Al2O3 supported
catalysts with 35% increase in CO conversion observed for CS cata-
lysts. The C6+ selectivity increases by 4.5% for SDA-CS catalysts, but
only a marginal increase of 0.7% is seen for Al2O3-CS catalysts. The
FT activity of IWI and CS catalysts measured as the rate of CO con-
sumption per unit mass of catalysts is shown in Fig. 11. The
Fig. 8. XPS spectra of SDA supported cobalt catalysts.
Fig. 9. XPS spectra of Al2O3 cobalt catalysts.
Table 4
XPS summary.
Catalyst B.E. Co-2p3/2 (eV) Spin-orbital
splitting (eV)
I-Co2p3/2/
I-shake-up
ICo/IAl
SDA-IWI 779.9 15.3 0.9 0.6
SDA-CS 779.7 15.2 1.1 1.6
Al2O3-IWI 780.4 15.2 0.7 0.2
Al2O3-CS 779.9 15.2 0.9 1.1
Co3O4-std 779.9 15.0 1.8 –
Fig. 10. CO conversion and selectivity for the synthesized catalysts.
86 S.S. Ail, S. Dasappa / Energy Conversion and Management 116 (2016) 80–90

9. increased performance of CS catalysts is attributed to its reduced
cobalt crystallite size, increased degree of reduction and higher
metal dispersion. A 2% decrease in the CH4 selectivity is observed
for SDA-CS catalyst compared to SDA-IWI catalyst while a 2.5%
increase in CH4 selectivity is observed for Al2O3-CS catalysts com-
pared to Al2O3-IWI catalyst. The CO2 selectivity varied from 1.1%
for SDA-CS catalysts to a maximum of 3.3% for Al2O3-IWI catalysts.
As seen in Fig. 10, the SDA supported cobalt catalyst displays
higher selectivity for C2–C5 hydrocarbon fraction as compared to
the Al2O3 supported cobalt catalyst. The C2–C5 selectivity for
SDA-IWI and SDA-CS catalyst is 2.4 times higher than Al2O3-IWI
and Al2O3-CS catalyst. The higher selectivity of C2–C5 hydrocarbons
could be due to the existence of alumino-silicate structure occur-
ring in the supports, giving it a zeolite-like behavior. A similar
observation was reported by Fraenkel and Gates [54]. The higher
C2–C5 selectivity for SDA supported catalysts results in lower C6+
selectivity, compared to Al2O3 supported catalysts.
The most striking feature observed is the FT product spectrum
for the synthesized catalysts. The HC product distribution is
observed to be a function of not only the synthesis procedure but
also a function of the support material used. Figs. 12 and 13 show
the weight fraction distribution of the liquid hydrocarbon products
(C10–C24) and waxes (C24+). The magnitude of active sites deposited
over the supported catalysts and the degree of reduction are crucial
factors that dictate the catalyst response for FT reaction. For SDA
supported catalysts, the C24+ weight fraction increases from
0.01 wt.% for combustion synthesized catalysts to 0.6 wt.% for
IWI catalysts. For Al2O3 supported catalysts, the C24+ weight frac-
tion increases from 0.67 wt.% to 0.8 wt.%. The hydrocarbon chain
growth probability factor, a, as described by the Anderson–Schul
z–Flory (ASF) distribution model was determined for the synthe-
sized catalysts based on the weight fraction of the various hydro-
carbon species. The a value was evaluated from the ASF plot in
the C10–C24 hydrocarbon range (Fig. 12), as described in [55]. a
increases from 0.89 for SDA-IWI catalysts to 0.94 for SDA-CS cata-
lysts. For Al2O3 supported catalysts, a increases from 0.92 for IWI
synthesized catalysts to 0.96 for CS catalysts. These values support
the increased higher-hydrocarbon chain growth on the CS cata-
lysts. The a values are comparable with the catalysts used in the
Shell middle distillate synthesis process (a = 0.92–0.93) and with
the Sasol’s fixed bed reactor (a ¼ 0:95) [56,57]. The experimental
results clearly indicate that the hydrocarbon chain growth proba-
bility increases for catalysts synthesized by solution combustion
Fig. 11. FT activity of IWI and CS catalysts; P = 3 MPa, T = 503 K, WHSV = 2610 ml/
(hÁgcat).
Fig. 12. Weight fraction of C10–C24 hydrocarbon product distribution for the synthesized catalysts.
Fig. 13. Weight fraction of C6–C24+ hydrocarbons in the FT products for the
synthesized catalysts.
S.S. Ail, S. Dasappa / Energy Conversion and Management 116 (2016) 80–90 87

10. method, primarily due to the increased metal dispersion and
higher catalyst reducibility.
Distinctly, the hydrocarbons synthesized over Al2O3 supported
cobalt catalysts resulted in the formation of heavier waxes than
middle distillates as compared to catalysts synthesized over SDA
supports. The HC product spectrum for Al2O3-CS catalysts show
0.8 wt.% C24+ compared to 0.6 wt.% C24+ for SDA-CS catalysts. The
33% higher weight fraction of C24+ hydrocarbons for Al2O3 sup-
ported cobalt catalysts is due to the total number of acidic sites
present in the SDA supports. The addition of silica to alumina
results in the formation of stronger Bronsted acid sites, through
bridged hydroxyl group formation across aluminum and silicon
atoms [17]. Lower surface acidity results in FT product spectrum
consisting primarily of lighter and more branched hydrocarbons
[18,58–60]. NH3-TPD of SDA supports, as seen in Table 2, show sur-
face acidity of 0.7 mmol NH3/gcat compared to 0.4 mmol NH3/gcat
for alumina supports, further supporting the acidic influence on
the FT product spectrum.
The stability of the synthesized catalysts is evaluated by observ-
ing the activity and selectivity changes during the experimental
run for a duration of 85 h. Fig. 14 shows the variation in CO conver-
sion and hydrocarbon selectivity as a function of time on stream. In
case of SDA supported catalysts synthesized by IWI and CS, the CO
conversion, and HC selectivity remain steady for a duration of 85 h,
indicating no possible catalyst deactivation. In case of Al2O3 sup-
ported catalysts synthesized by IWI method, the CO conversion
drops from 31.4% to 19.6% indicating a 38% reduction in FT activity
over a duration of 85 h, while a 10% reduction in the FT activity is
recorded for Al2O3-CS catalysts. The observed spectrum in Fig. 14
evidence that the combustion synthesized catalysts and silica
doped alumina supports prevent catalyst deactivation as seen in
Al2O3-IWI catalyst.
Fig. 15 compares the FT activity and product yield for the cata-
lysts synthesized in this work. The SDA supported combustion syn-
thesized catalysts show highest C6+ yield corresponding to
0.31 g-C6+/g-syngas, at a space velocity of 2610 ml/(hÁgcat). Overall,
an improved FT performance is achieved with catalysts synthe-
sized in this work. Fig. 15 clearly signifies an increase in the C6+
productivity for CS catalysts compared to IWI catalysts, with nearly
39% increase for CS-Al2O3 catalysts and $70% increase for CS-SDA
catalysts. While SDA-IWI catalysts produce a maximum concentra-
tion of liquid fuel fractions (C10–C24) compared to the other syn-
thesized catalysts, the highest yield of higher hydrocarbons, that
include liquid fuel and wax, is achieved by SDA-CS catalysts. With
mild hydrocracking of waxes the yield of high quality diesel frac-
tions can be greatly enhanced using SDA-CS catalysts.
4. Conclusions
The combustion synthesised cobalt catalysts show enhanced
catalytic properties compared to the conventionally synthesized
IWI catalysts, with respect to its reduced crystallite size, higher
degree of reduction and higher metal dispersion. The primary rea-
son of high metal dispersion for CS catalysts is attributed to
reduced degree of metal sintering during the synthesis step, which
is a common occurrence for IWI catalysts. Strikingly, the CS cata-
lysts exhibit reduced fractions of cobalt aluminate formation that
otherwise hider the degree of reduction resulting in decreased
fraction of active cobalt sites. Compared to CS catalysts, the IWI
catalysts reveal higher concentrations of the irreducible cobalt alu-
minates that contribute to its reduced dispersion and hence
reduced FT activity. Moreover, while the silica doping of the alu-
mina support is known to impede the formation of cobalt alumi-
nates, the combined use of combustion synthesis method and the
silica doped alumina support resulted in the formation of cobalt
catalysts with improved properties, facilitating higher FT activity
and higher C6+ selectivity. The investigations of the FT product
spectrum revealed higher fractions of waxes (C24+) with Al2O3
supported cobalt catalysts while hydrocarbons ranging from C10–
Fig. 14. CO conversion and selectivity variation with time on stream for the
synthesized catalysts; WHSV = 2610 ml/(hÁgcat), T = 503 K, P = 3 MPa.
Fig. 15. C6+ yield comparison for the synthesized catalysts. WHSV = 2610 ml/
(hÁgcat), P = 3 MPa, T = 503 K.
88 S.S. Ail, S. Dasappa / Energy Conversion and Management 116 (2016) 80–90

11. C20 formed as the primary product for SDA supported catalysts. The
differences in the nature of product spectrum is associated with
the variation in the acidic characteristic of the Al2O3 and SDA sup-
ports – alumina being more basic compared to SDA, favoured the
formation of waxes while SDA supported catalysts promoted the
formation of middle distillates. Also, the hydrocarbon selectivity
over CS catalysts resulted in higher wax formation compared to
IWI catalysts, implying higher chain growth probability over CS
catalysts, than over IWI catalysts. The overall hydrocarbon yield,
measured as gC6+/gsyngas, increases by a margin of 40–70% with
the use of CS catalysts.
Acknowledgements
The authors wish to acknowledge financial support of Ministry
of New and Renewable Energy (MNRE), Government of India and
also the instrumentation facilities provided by National Centre
for Combustion Research and Development (NCCRD), Centre for
Nano Science and Engineering (CeNSE) at IISc Bangalore and
National Environmental Engineering Research Institute (NEERI),
Nagpur. Further, the authors would like to express their gratitude
to Sasol Germany GmbH for providing the support materials used
in this work.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.enconman.2016.
02.075.
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